Commercially available, off-the-shelf capacitors simply cannot meet the extreme requirements of current and future industrial or military applications for intense, transient sources of high voltage pulse power. Furthermore, minimizing the component volume and weight are critical to the viability of new power storage systems for use in aircraft, spacecraft, and other mobile devices.
Existing dielectric capacitors have quite low energy densities, both on volume and mass basis. Electrochemical capacitors, including double layer capacitors and supercapacitors, offer high energy and power density, but their rate capability is limited by mass transfer and faradaic reaction rates. No current capacitor technology has the combination of energy density, power density, and rate capability required for new systems currently under development or envisioned for the future.
The maximum volumetric energy density W (J/cm3) stored by a dielectric capacitor,W=0.5∈0∈rE2bd   (1)depends on the relative dielectric permittivity (or dielectric constant, ∈r) and dielectric breakdown field strength (Ebd in V/μm). For a parallel plate capacitor with area A, thickness d, and capacitance C=∈0∈rA/d, we have the alternate expressionW′≡AdW=0.5Cd2E2bd=0.5CV2bd   (2)for the maximum energy W(J) stored by a capacitor charged to the breakdown voltage Vbd. The most obvious way to increase W′ (or W) would be to choose dielectric materials with the highest possible breakdown field strength. Many polymers not only have high values of Ebd, but the also offer the additional advantage of processability. Unfortunately, the dielectric constants of most polymers are negligible.
The energy density could also be increased by blending high-∈r inorganic ceramic materials into polymers, leading to higher effective dielectric constant. Many groups have attempted to disperse commercially available, high-∈r ceramic oxides, such as barium titanate (BaTiO3) into polymers followed by fabrication of thin films. Unfortunately, both experiment and theory show that the inorganic loading must be quite high to significantly increase the effective dielectric constant. For example, the symmetric Bruggeman equation, one of several effective medium theories, suggests that increases in ∈eff will not be observed until filler loadings are greater than 30% by volume. Conversely, there have also been many studies that show the Bruggeman equation grossly overestimates ∈eff and that achieving high ∈eff values requires inorganic ceramic particle loadings greater than 50% by volume.
A major problem that develops from high inorganic loadings in polymers (particularly BaTiO3) is poor dispersion in the polymer matrix. The poor dispersion of inorganic in polymer leads to poor ∈eff and poor Ebd caused by the domination of the Ebd of the defect-rich inorganic filler network. The recent work by Kim et al. has shown that surface modification of BaTiO3 by various organo-phosphonic acids leads to better dispersion of BaTiO3 particles in the polymer matrix, to a high effective dielectric constant, and to only about a 50% decrease in breakdown field strength compared to polymer alone.
The recognition that polymer-filler interfaces can dominate capacitor performance leads naturally to the concept of polymer nanocomposite dielectrics. As such, a need exists for improved dispersion of inorganic loadings in a polymer matrix.